Process of recovering zinc metals and its alloys from zinc dross



United States Patent PROCESS OF RECOVERING ZINC METALS AND ITS ALLOYSFROM ZINC DROSS Henry C. Deterding, Kansas City, M0.

N0 Drawing. Application September 22, 1951, Serial No. 247,897

3 Claims. (Cl. 75-24) This invention relates to a flux composition andto methods of making and using the same. More particularly, theinvention relates to a flux composition for use in the refining of zincand zinc base alloys from mixtures thereof with metallic oxides.

The commercial production of high purity zinc (99.99% grade) has madepossible the development of various zinc base alloys having desirablephysical properties and excellent casting characteristics. While sometypes of zinc base alloys used extensively are cast in molds undergravity pressure, the widest application of the zinc base alloy is inconnection with high pressure die casting methods. In that case, theZinc alloy in molten form is forced into cavity dies under highpressure. The speed of casting, degree of pressure and size of the entrygates actually result in the alloy entering the cavities as a spray ofthe molten alloy, with the result that oxidation of some of theconstituents of the alloy takes place. In a subsequent operation, theuseful castings are removed from the cast forms and the remaining metalin the form of sprues, gates, runners, defective castings, and the likeare remelted.

On the remelting of the scrap, it is an observed fact, peculiar to zincbase alloys, that the oxides present do not separate cleanly from themetal. Instead, the oxides trap or hold as much as several times theirown weight of metal, forming a thick mushy dross that floats upon thesurface of the molten metal. In the preparation of zincaluminum alloys,a similar dross is formed as a result of the agitation required to alloythe aluminum with the 21110.

Several methods of metal recovery from the dross are in current usage.In some plants the dross is skimmed from the metal, cast in convenientform and returned to zinc smelters for recovery by distillation methods.Other die casting plants flux the dross to recover whatever metal can bereclaimed. Either method represents a substantial economic loss to thedie casting plant. Where fluxes are used, they commonly consist of zincchloride, ammonium chloride or the double salt: zinc ammonium chloride.These salts may be used singly or in combination with varying amounts ofother salts to reduce fumes. Such fluxes are not entirely satisfactorybecause of low metal recovery and because of the creation ofobjectionable fumes and dust due to the volatility of the fluxes.

In accordance with my present invention, I provide a relatively lowmelting point flux composition which does not attack the metallic zinc,itself, but separates the oxides from the metal holding them insuspension. The fluxoxides mixture that results floats on the surface ofthe molten metal and is skimmed therefrom as a thick paste. While theflux composition of my invention is a relatively low melting pointmixture, having a freezing point not in excess of about 950 F., andpreferably between 815 and 850 F., the constituents of my fluxcomposition are so chosen that there is substantially no fuming orsmoking of the flux composition during its use.

It is therefore an important object of this invention to provide a fluxcomposition having a relatively low freezing point and suitable for usein the refining of zinc mixtures of zinc metal and metallic oxides, theconstituents of which are sufficiently non-volatile that the compositiondoes not smoke or fume substantially during use thereof for refiningpurposes.

It is a further important object of this invention to provide a fluxcomposition mainly composed of a eutectic mixture of metal chlorides andcontaining an effective amount of an alkaline earth fluoride to improvethe efficiency of the flux composition in its refining action.

It is a further object of this invention to provide an effective methodof refining zinc and zinc base alloys from admixtures thereof withmetallic oxides by the use of an inorganic, non-fuming flux.

Other and further important objects of this invention will becomeapparent from the following description and appended claims.

My flux composition comprises, broadly, two components: one, a majorcomponent comprising a mixture of metal chlorides either largely orwholly in the form of a eutectic composition; and, the other a minorcomponent comprising an alkaline earth fluoride selected from the groupconsisting of calcium, strontium and barium fluorides. The firstcomponent comprises calcium chloride, sodium chloride and bariumchloride. The proportions of the ingredients of the first, or majorcomponent of my flux composition are as follows:

Per cent by weight 4 55 Calcium chloride 0 Barium chloride 2545 Sodiumchloride 10-30 Preferably, the first component of my flux compositioncomprises a eutectic mixture of calcium chloride, sodium chloride andbarium chloride, the specific composition of which is as follows:

Per cent Calcium chloride 48 Sodium chloride 21 Barium chloride 31 Thiseutectic mixture has a freezing point of about 830 alkaline earthfluoride, or a mixture of alkaline earth fluorides. The alkaline earthmetals found to be satisfactory are calcium, strontium and barium. Onthe basis of parts by weight of the first component, the secondcomponent comprises an amount of calcium, barium or strontium fluorides,or mixtures thereof, equivalent stoichiometrically to from 1 to 5 partsby weight of calcium fluoride. Preferably, 3 parts by weight of calciumfluoride for each 100 parts by weight of the first component are used,but if barium fluoride is substituted for calcium fluoride, theequivalent, or 6% parts, of barium fluoride are employed. Thestoichiometrical proportions of calcium fluoride, strontium fluoride andbarium fluoride are l to 1.6 to 2.25, approximately. The amount of thealkaline earth metal fluoride, or mixture of fluorides, equivalent tofrom 1 to 5% of calcium fluoride by weight of the first component can becalculated from the stoichiometric proportions. In general, an amount ofthe second component should be used such that the freezing point of theresulting mixture is not over 950 F., and, preferably, not over 850 F.

In making up my flux composition, the first component is prepared atfusion temperatures and then the second component added thereto. Forinstance, if the preferred eutectic mixture of 48% calcium chloride, 21%sodium chloride and 31% barium chloride is used, the mixture is heatedto a temperature of about 1000 F., or higher, and while in fused, ormolten, condition, there are added to such mixture 3 parts by weight ofcalcium fluoride for each 100 parts of the molten mixture, or 4.8 partsof strontium fluoride, or 6% parts of barium fluoride. The melt is thenstirred by any suitable agitating means until all of the fluoride saltsare dissolved. The molten mass may be cast in pellets, or it may be castin any suitable form, and then cooled and reduced to convenient particlesize by crushing, grinding or the like. Where the first componentcomprises the preferred eutectic mixture aforesaid plus 3 parts ofcalcium fluoride for each 100 aromas parts of the eutectic mixture, thefreezing point of the flux composition is about 830 F.

Instead of making up the first component separately and then adding thesecond component to it, it is possible to use a mixture of theingredients of the first and second components, without pre-fusing theingredients of the first component. However, substantially higher metaltemperatures are required, and, further, the prefusing of the firstcomponent is a distinct advantage in that it starts to melt at itseutectic melting point.

Up to the equivalent of 3% of calcium fluoride by weight of the firstcomponent is substantially Without influence on the freezing point ofthe eutectic mixture, but as the amount of calcium fluoride, or itsequivalent of the other fluorides, is added, the freezing point rises.EX- perimental data on additions of calcium fluoride to the preferredeutectic mixture constituting the first component are as follows:

F. freezing pgint The above listed freezing points were determined bythe use of a commercial grade pyrometer, but it should be pointed outthat freezing points are not sharply defined and exact determinationsvary with the judgment of the observer. Since the change in freezingpoint is governed by the fluoride radical and not the metal radical, thechanges in freezing point are roughly inversely proportional to themolecular weights of the fluorides used. Thus, almost double the amountof barium fluoride may be substituted for calcium fluoride to effect thesame increase in freezing point temperature. No advantage is gained bythe use of strontium or barium fluoride over the use of calciumfluoride, and the cost in each case is appreciably higher. Consequently,calcium fluoride is preferred.

Although the preferred composition of the first component is theeutectic composition previously given, it is possible to vary thepercentages a few per cent either way from the preferred compositionwithout noticeable changes in the freezing point. Thus, any specificcomposition within the broader composition heretofore given, which has afreezing point not over 950 F. and preferably between 815 F. and 850 F.can be used.

In the use of my flux composition, the molten metal and its dross, asfor instance zinc metal and zinc base alloys and zinc dross inadmixture, are heated to a temperature of approximately 900 F., and anamount of the fused flux composition approximately equal to of theweight of the dross is added to the surface of the molten mass.

The flux is preferably allowed to melt, or at least to preheatthoroughly, and then is stirred or rabbled into the mass of dross. Agradual separation of oxides from the metal occurs with a progressivethickening of the fused flux mass as the oxide content increases. If thedross is of high oxide content, more flux should be added to effectproper consistency and effect clean separation of the metal from thedross. In general, one part by weight of flux will take care of twoparts by weight of metallic oxides. When a satisfactory separation hasbeen made, the oxide-saturated flux is skimmed from the surface of themetal. The percentage recovery of the zinc or zinc base alloy isnormally between 75 and 80% theory, as compared with around 50 to 60% byheretofore known methods.

Under some conditions, as for instance where the dross has a high ironcontent and this iron is present as an ironaluminum compound, the flux,if within the above specified formulas, will not react properly with thedross. If it is known that the dross contains iron, or previousexperrence with a similar dross indicates that the dross is given apreliminary treatment by rabbling with a small amount of a suitableoxidizing agent, such as sodium or potasslum nitrate in the ratio ofabout 0.1 to 1.0% by weight of nitrate to weight of dross, and thentreated with the flux of my composition in the manner already descrlbed.In iron-bearing drosses so treated, a substantially larger ratio of fluxto dross is required, say up to 25% of flux by weight of the dross.These iron-bearing drosses are not usually encountered in the melting ofgates, sprues and the like, but are more likely to be encountered in thecase of drosses formed by alloying alurrlllinum with the zinc in thepreparation of zinc base a oys.

The flux composition of my preferred formula has been heated to 1300 F.without any fumes being observed. This is a higher temperature than thatat which my flux composition would normally be used in the refining ofzinc dross. Consequently, my flux composition may properly be said to besubstantially non-fuming and nonsmoking under normal operatingconditions.

The treatment of iron-bearing drosses is preferably a two-step process,as described above, in which the dross is first treated with thenitrate, or nitrates, and then with the flux of my composition.Iron-bearing drosses are only infrequently encountered and should notoccur in good zinc base alloy shop practice. The nitrates cannot beadded in the preparation of my flux, since they break down at the fusiontemperature, around 1000 F., that is required to form the eutecticmelting point composition.

My method of refining by the use of the flux composi tion hereindescribed is particularly adapted for the refining of zinc and zinc basealloys. By the term zinc base alloys is meant any zinc alloy containingat least 85% of zinc. Usually a zinc base alloy contains aluminum, withor without copper and magnesium, plus the usual impurities. Zinc basealloys commonly used for die casting and known to the trade as Zamak aredescribed in The American Society For Testing Materials SpecificationB24049T.

It will be understood that modifications and variations may be effectedwithout departing from the scope of the novel concepts of the presentinvention.

I claim as my invention:

1. The method of recovering zinc metal and zinc base alloy from a zincdross containing zinc metal, zinc base alloys and oxides of metalspresent in said zinc base alloys, which comprises agitating said drossin a molten state at a temperature of around 900 F. with an admixture ofa flux composition consisting essentially of a mixture of from 40 to 55%of calcium chloride, from 25 to 45% of barium chloride, a sufficientamount of sodium chloride to make a total of and a sufficient amount ofan alkaline earth metal fluoride selected from the group consisting ofcalcium, strontium and barium fluorides to be stoichiometricallyequivalent to between 1 and 5% calcium fluoride by weight of saidmixtures, continuing such agitation until a substantial separation ofmolten zinc metal and zinc base alloys from the metallic oxides has beeneffected, with the metallic oxides held in suspension in said flux ontop of said molten zinc metal and zinc base alloys, and skimming offsaid flux-metallic oxide suspension to recover zinc and zinc base alloyssubstantially free from metallic oxides.

2. The method of recovering zinc metal and zinc base alloy from a zincdross containing zinc metal, zinc base alloys and oxides of metalspresent in said zinc base alloys, which comprises agitating said drossin a molten state at a temperature of around 900 F. with an admixture ofa flux composition consisting essentially of a eutectic mixture of about48% of calcium chloride, 21% of sodium chloride and 31% of bariumchloride, and an amount of an alkaline earth metal fluoride selectedfrom the group consisting of calcium, strontium and barium fluoridesstoichiometrically equivalent to between 1 and 5% of calcium fluoride byweight of said eutectic mixture, continuing such agitation until asubstantial separation of molten zinc metal and zinc base alloys fromthe metallic oxides has been effected, with the metallic oxides held insuspension in said flux on top of said molten zinc metal and zince basealloys, and skimming off said flux-metallic oxide suspension to recoverzinc and zinc base alloys substantially free from metallic oxides.

3. The method of recovering zinc and zinc base alloys from a zinc drosscontaining zinc metal, zinc base alloys and oxides of metals present insaid zinc base alloys, which comprises agitating such zinc dross in amolten state in the presence of a flux of the composition consistingessentially of a mixture of from 40 to 55% of calcium chloride, from 25to 45% of barium chloride, a suflicient amount of sodium chloride tomake a total of 100%, and a suflicient amount of an alkaline earthfluoride selected from the group consisting of calcium, strontium andbarium fluorides to be stoichiometrically equivalent to between 1 and 5%of calcium fluoride by weight of said mixture, said flux being in theproportion of about 1 part of flux to 2 parts by weight of metallicoxides present, continuing such agitation until a substantial separationof molten zinc metal and zinc base alloys from said metallic oxides hastaken place, with said metallic oxides held in suspension in said fluxfloating on said molten zinc metal and zinc base alloys so separated,and skimming ofi the flux-metallic oxide suspension to recover saidmolten zinc metal and zinc base alloys substantially free from metallicoxides.

References Cited in the file of this patent UNITED STATES PATENTS 79,701Stevens July 7, 1868 2,148,664 Wile Feb. 28, 1939 2,472,025 Peake May31, 1949 2,474,674 Holden June 28, 1949 FOREIGN PATENTS 450,690 GreatBritain July 23, 1936 114,482 Australia Dec. 29, 1941 OTHER REFERENCESMetals and Alloys, vol. 5, page 561, December 1934. Handbook ofNon-Ferrous Metallurgy, by Liddell, vol. 2, 2nd ed., pages 208, 430.Published 1945.

1. THE METHOD OF RECOVERING ZINC METAL AND ZINC BASE ALLOY FROM A ZINCDROSS CONTAINING ZINC METAL, ZINC BASE ALLOYS AND OXIDES OF METALSPRESENT IN SAID ZINC BASE ALLOYS, WHICH COPRISES AGITATING SAID DROSS INA MOLTEN STATE AT A TEMPERATURE OF AROUND 900* F. WITH AN ADMIXTURE OF AFLUX COMPOSITION CONSISTING ESENTIALLY OF A MIXTURE OF FROM 40 TO 55% OFCALCIUM CHLORIDE, FROM 2.5 TO 45% OF BARIUM CHLORIDE, A SUFFICIENTAMOUNT OF SODIUM CHLORIDE TO MAKE A TOTAL OF 100%, AND A SUFFICIENTAMOUNT OF AN ALKALINE EARTH METAL FLUORIDE SELECTED FROM THE GROUPCONSISTING OF CALCIUM, STRONTIUM AND BARIUM FLUORIDES TO BESTOICHIOMETRICALLY EQUIVALENT TO BETWEEN 1 AND 5% CALCIUM FLUORIDE BYWEIGHT OF SAID MIXTURES, CONTINUING SUCH AGITATION UNTIL A SUBSTANTIALSEPARATION OF MOLTEN ZINC METAL AND ZINC BASE ALLOYS FROM THE METALLICOXIDES HAS BEEN EFFECTED. WITH THE METALLIC OXIDES HELD IN SUSPENSION INSAID FLUX ON TOP OF SAID MOLTEN ZINC METAL AND ZINC BASE ALLOYS, ANDSKIMMING OFF SAID FLUX-METALLIC OXIDE SUSPENSION TO RECOVER ZINC ANDZINC BASE ALLOYS SUBSTANTIALLY FREE FROM METALLIC OXIDES.